1. Field of the Invention
This invention relates to a method and apparatus for communications with electromagnetic waves. The system preferably operates at millimeter-wave frequencies and uses polarization diversity.
2. Description of Prior Art
The information transmission capacity of a communications system can be substantially increased with the use of polarization diversity. This is true for both one-way and two-way communications systems. Vertical and horizontal polarizations are often used in satellite communications and other point-to-point microwave links to isolate both transmit and receive signals, or to increase information capacity.
For local communications systems using a millimeter-wave carrier, polarization cross-talk caused by precipitation is a commonly encountered problem for systems using dual linearly polarized signal transmission. Further, where a transmission link involves successive reflections by buildings and other objects, as in an urban environment, considerable variations in the polarization state of the signals occur, making signal isolation by orthogonal polarization less effective.
For a given frequency, a circularly polarized plane or quasi-plane electromagnetic wave propagating in open space may have its field vectors revolving clockwise (CP), or counterclockwise (CCP). Two such waves, rotating oppositely with respect to each other, are orthogonal to each other and may be isolated with proper antenna feeds and electronic circuitry. However, precipitation and/or reflection/diffraction from buildings and other obstacles can distort the waves and cause elliptical polarization. If the waves become excessively elliptically polarized, the information carried by the waves cannot be retrieved.
U.S. Pat. No. 4,747,160 teaches a low power multi-function cellular television system capable of two-way communication services. An omnidirectional transmitter transmits vertical and horizontal linear polarized waves. The system taught by the '160 patent preferably operates in the 27.5 GHz to 29.5 GHz millimeter wave band.
U.S. Pat. No. 4,264,908 teaches a polarization correction network that automatically compensates for cross polarization caused by, for example, precipitation. The network transmits vertical and horizontal linearly polarized waves.
U.S. Pat. No. 4,106,015 discloses a radar system that eliminates rain echo signals. Pulsed, polarized waves are transmitted, and two separate receiving channels receive orthogonal components of a rain echo signal. The rain echo signal is eliminated by adjusting an amplitude of the orthogonal components of the rain echo signal, and then adjusting the phase of the signals to be opposite with respect to each other.
U.S. Pat. No. 4,737,793 discloses a dual-polarized microstrip antenna capable of simultaneously transmitting mutually orthogonal polarizations, including clockwise and counterclockwise circularly polarized waves, to double the capacity of a given frequency band.
U.S. Pat. No. 4,146,893 teaches a satellite communications system which compensates for polarization distortion caused by precipitation and incomplete polarization characteristics of antennas by predistorting a circularly polarized wave to an elliptically polarized wave. As the elliptically polarized wave encounters the depolarizing medium, a circular wave forms and is received by the satellite.
U.S. Pat. No. 3,956,699 discloses an electromagnetic wave communications system which transmits and receives waves having mutually orthogonal polarizations. The system provides polarization control prior to power amplification when transmitting, and subsequent to amplification when receiving.
U.S. Pat. No. 5,337,058 teaches a fast switching lens which is positioned in front of a radar antenna to manipulate the polarization of a transmitted wave to various polarizations. The lens can also receive waves of various polarizations.
U.S. Pat. No. 4,329,687 discloses a radar system which alternately radiates clockwise and counterclockwise circularly or elliptically polarized waves. A relatively high signal-to-clutter ratio is achieved by analyzing the phase differences between the two orthogonal components of the transmitted wave and the phase differences of the two orthogonal components of the received wave.
The prior art references discussed above fail to disclose a method or apparatus for a communications system that can restore circular polarization to a distorted wave, and can operate in an urban environment at the millimeter-wave frequencies. Thus, it is apparent that a communications system that operates in the millimeter-wave frequencies, provides dual polarization, and achieves relatively high signal restoration and isolation is needed.